The present invention relates to a safety device for power driven chain saws and more particularly to a safety device for preventing sudden inadvertent rotation of the power driven chain saw which might otherwise displace the cutting chain assembly from the workpiece being cut in a direction towards the operator of the power driven chain saw.
Compact and portable power driven chain saws have become extremely popular in recent years for various cutting purposes, particularly when an extremely large workpiece, such as a tree trunk, is to be cut. The power driven chain saws are typically used outdoors in varying weather conditions. Furthermore, the power driven chain saws are typically used to cut workpieces which are so bulky as to render it impractical or impossible to cut such workpieces using a manual tool or a table saw. Typically the workpiece is self-supported and, accordingly, may move in an uncontrollable and in an unpredictable manner as the cutting process progresses. The conditions inherent in the typical uses of a power driven chain saw result in situations which are prone to result in various types of accidents which cause damage to the workpiece or the chain saw, or cause injury to the operator. Furthermore, the very portability which makes the power driven chain saw an attractive cutting tool contributes to careless and inattentive use of the device, which further contributes to accidents.
The need for safety devices in conjunction with power driven chain saws is well recognized in the art. Various methods and apparatuses have been described in previous patents and have been included in power driven chain saws currently commercially available. Such safety devices approach the safety hazards associated with power driven chain saws by shielding the cutting chain or by providing redundant controls or various braking devices to instantaneously stop the operation of the cutting chain assembly.
One of the most common safety devices offered for power driven chain saws is a cutting chain guard to shield all or part of the cutting chain assembly so as to avoid injury to the operator. Such cutting chain guards include removable guards disclosed, for example, in U.S. Pat. No. 2,826,294, issued Mar. 11, 1958, to Nicodemus. While such cutting chain guards protect the operator to varying degrees, they tend to interfere with the operation of the chain saw for some purposes. Furthermore, in order to accommodate the operation of the chain saw for these purposes, the cutting chain guard may be removed from the power driven chain saw, thereby defeating its safety purpose. Furthermore, because of the inconvenience involved in removing and reinstalling cutting chain guards to accommodate varying circumstances, it is not uncommon for a cutting chain guard, once removed, to remain permanently removed from the power driven chain saw.
Another approach that has been taken in the past to provide some protection to the operator, the workpiece, and the power driven chain saw during a kickback condition is to provide an inertia actuated safety brake, which responds to a sudden acceleration or rotation of the power driven chain saw by disengaging the motor of the power driven chain saw and/or engaging a brake to stop the cutting chain assembly from moving. An example of a power driven chain saw using an inertia actuated safety device is disclosed, for example, in U.S. Pat. No. 3,485,326, issued Dec. 23, 1969, to Wilkin.
Still another approach taken to reduce the dangers associated with kickback of power driven chain saws is to provide a brake lever adjacent the one of the two handles of the power driven chain saw which is closer to the cutting chain assembly. The brake lever is selectively pivotable between a position adjacent the handle and a position adjacent the cutting chain assembly. A brake for stopping the motion of the cutting chain assembly is actuated when the brake lever is disposed in the latter position. In operation, a power driven chain saw with a brake lever is typically held by both hands of an operator, one of the hands engaging the handle adjacent the brake lever. During a kickback condition, the one hand adjacent the brake lever pivots the brake lever to stop the motion of the cutting chain assembly. An example of such a device is disclosed, for example, in U.S. Pat. No. 3,664,390, issued May 23, 1972, to Mattsson. While such a safety device does, in many cases, effectively stop the cutting motion of the cutting chain assembly in the event that the power driven chain saw is held properly, no provision is made in any of these devices for arresting the rotational motion of the power driven chain saw away from the workpiece. Furthermore, no provision is made to ensure that the power driven chain saw is, in fact, held properly so as to assure that the brake lever will, in fact, be actuated in the event of a kickback condition.
Finally, in U.S. Pat. No. 3,361,165, issued Jan. 2, 1968, to Irgens, a power driven chain saw is disclosed which is adapted to be held by an operator using two hands wherein two controls are provided, one associated with each hand, to assure that the power driven chain saw was held properly by both hands. A switch associated with one hand is a finger trigger switch operable to control the throttle. The handle associated with the other hand is itself a control. The handle is pivotally mounted to the housing of the power driven chain saw so as to be pivotable to selectively engage a clutch which is mechanically interposed between the motor of the power driven chain saw and the cutting chain assembly thereof. Thus, both controls are required to be in an operative position in order to obtain a cutting action by the cutting chain assembly. However, no provision is made in Irgens for preventing inadvertent rotation of the power driven chain saw as a result of kickback, or for braking the momentum of the cutting chain assembly, once it is set in motion.
The amount of torsional force required to stop the inadvertent rotation of a power driven chain saw during a kickback condition is typically very small. Nonetheless, even where one of the various above described prior art safety devices are provided to decelerate or brake the cutting chain assembly, it is still possible for the operator, the workpiece, or the power driven chain saw itself to be damaged. Accordingly, what is needed is a safety device for a power driven chain saw, and a power driven chain saw incorporating such a safety device wherein inadvertent rotation of the cutting chain of the power driven chain saw away from a workpiece and towards the operator is directly inhibited.